Remove the check for union types on match error

[sylvanc]

A nice attempt to produce a much friendlier error message when a union
type pattern is involved and a match expression doesn't type check is
sadly removed in this commit.

Unfortunately, it assumed union types would be composed of nominal types
(not true), and that the problem with the match was a union type with
varying capabilities (this is actually allowed).

A nicer error message would indeed be useful here. The right approach
would be to emit errors in matchtype.c when they are needed, i.e. on
DENY.

[jemc]

@sylvanc, can we use a more explanatory message than "this capture violates capabilities"?

In #1545 we discussed using something like "this capture type has a mismatch in reference capabilities, which can't be differentiated at runtime", though I'm still open to improvements on that.

[sylvanc]

We can, but it should be done as part of moving the error generation into matchtype.c, to get more accurate messages. This is just a "rollback" PR.